The proposed mechanistic formula feeding study sets out (i) to identify the mechanism(s) by which an extensively hydrolyzed casein formula is able to protect children at risk for type 1 diabetes (T1D) from beta-cell autoimmunity and (ii) to assess whether partly or extensively hydrolyzed whey formulas have mechanistic characteristics common with the casein formula. Based on clinical and experimental observations the study will focus on defining the effects of four different formulas on intestinal permeability, IL-17 immunity, serum metabolome, and gut microflora. The study will be based on a randomized pilot intervention trial using an intention to treat statistical analysis to compare e.g. gut permeability between the four treatment groups. We hypothesize that the highly hydrolyzed casein formula decreases intestinal permeability, down-regulates IL- 17 immunity and proinflammatory lysophoshatidylcholines, and stabilize Lactobacilli levels in the gut micro- flora when compared to the conventional cow's milk formula, whereas the whey formulas have more modest, if any, effects on gut permeability, IL-17 immunity, serum metabolome or intestinal microbiota. The study population comprises 200 newborn infants with HLA-conferred susceptibility to T1D. The mothers will be encouraged to exclusively breast-feed their infants as long as possible. The timing of weaning and introduction of study formula will be left to the mother. The infants are randomized to be weaned to one of four study formulas: i) standard cow's milk formula; (ii) a partially hydrolyzed whey formula; (iii) an extensively hydrolyzed whey formula; and (iv) an extensively hydrolyzed casein formula. The target will be that the infant should be exposed to his/her study formula for at least 90 days before the age of 270 days. The diet of the infant will be studied with 3-day food records at the age of 1, 3, 6, 9, and 12 months of age. To estimate the amount of formula used the weight of the infant will be measured just before and after each formula feed. The HLA genotype will be analyzed from cord blood, and the result will be available within 10 days after birth. The family will visit the Study Center when the infant is 1, 3, 6, 9, and 12-month-old. Blood samples will be obtained on each visit. In addition the families are asked to collect stool samples at home once a month during the study. Intestinal permeability will be assessed with the lactulose/mannitol test at the age of 3, 6, 9, and 12 months. Gut microflora will be analyzed with high-throughput, culture-independent methods and serum metabolome with established metabolomics platforms. Il-17 immunity will be studied using peripheral blood mononuclear cells. This proposal addresses broadly autoimmunity, one of the areas the current FOA solicits applications for. This work will generate novel knowledge of the disease process leading to overt T1D by studying potential mechanism(s) mediating the protective effect conferred by an extensively hydrolyzed casein formula against beta-cell autoimmunity. The identification of such mechanism(s) will most likely facilitate the refinement of effective preventive measures based on modifications of early infant nutrition. PUBLIC HEALTH RELEVANCE: Weaning to an extensively hydrolyzed casein formula has been shown to reduce the appearance of diabetes-predictive autoantibodies by half by the age of 10 years in children at risk for type 1 diabetes. Our study sets out to identify those mechanism(s) which mediate such a protective effect. Learning about the mediating mechanisms will generate novel knowledge of the disease process leading to clinical type 1 diabetes and facilitate the refinement of effective preventive measures based in modifications of early infant nutrition - a safe and relatively simple intervention.